I've been playing Harn for almost 20 years, so how about I give it a stab:
Harn is a game world produced by Columbia Games Inc. that was originally created back in 1983 (I think - that is as far back as I can remember it) by a Welsh writer and medieval historian named N. Robin Crossby. The actual word 'Harn' refers to a small island, kind of like England, to the NW of Lythia, the main continental landmass of the world called Kethira.
The main points of the world are this:
1. It is system neutral and has been designed to be system neutral from its inception. Use d20. Use Rolemaster. Use HarnMaster. Use Gurps. The world works with all of them, though some require minor modifications in order to keep the 'feel' of the game world.
2. It is a low magic, low creature world, also by design. The theory is that you can always add more magic and creatures when you need them, but it is devilishly hard to take them away instead.
3. It is very firmly based on feudal cultures from our own history. There are serfs, knights, barons and the societies all operate more or less like actual medieval cultures would have.
4. It is very logical and consistent in its design, meaning that all the little details that often get glossed over in game worlds are taken care of in Harn. Tectonic plates? Got em. Star map? Got it. Ocean current map? Yep. It is the most living and breathing world I have ever experienced in all my gaming career. It smells as if it actually exists somewhere.
5. The maps are utterly gorgeous, with the exception of some of the more recent sourcebooks where a new artist was used for the odd map. I saw a map for a city in Harn in a dragon magazine as a kid. That alone got me to buy my first copy of the world.
6. The writing is generally at a higher level than most RPG products, mostly because of the writer being a medieval historian - though this is hotly disputed here at EnWorld.
7. All written products assume a standard date - 720 TR - so that while there is a comprehensive and very believable history for the world to that point, the GM is free to write whatever future he or she desires.
The only really required books to have are:
Harnworld - a package set with the Harndex (index of all terms, people and places), the terrific map of the island of Harn, and Harnworld (Harnic history, world layout)
and
Harnplayer - a similar sourcebook with all the GM only tidbits removed. It is intended to be a desktop reference for the players. Trust me, with the level of detail Harn has, they will need it.
The island of Harn, being the main focus of the world, has been very well detailed through a series of kingdom modules. There are currently seven human kingdoms on Harn, one sindarin (elves) and one khuzdul (dwarves). Each one has a kindom module printed about the kingdom with more detailed info on the races and people inside.
As a recommendation, I would suggest the kingdom of Chybisa to start. It is a small kingdom, which when you are talking about the level of detail involved in Harn, is a very good thing. GMs new to gaming or even just new to the game world should consider Chybisa a must purchase with the above two books.
As I have mentioned, you can play on Harn with literally any game system on the market if you so choose. Columbia Games also makes a game system tailored to Harn called HarnMaster. It is a skill-based system (no classes) with graphic injuries (stab in the chest instead of a 7hp wound), realistic healing, etc. It is an excellent companion to Harn, the world, but there are quite a few people who have great campaigns running any manner of system.
If you are a d20 player, there are some d20 .pdf files designed to help ease the transition to Harn, and I highly suggest that you try this avenue before making the wholesale switch from one system to another. If you wish more information, there is the Harnforum (
http://www.harnforum.com/) where the harniacs meet, CGI's site that has already been mentioned, and Sword and Shields, which has also been mentioned.
